Protective installation for electric circuits



June 2, 1925. 1,540,679

C. E. BENNETT PROTECTIVE INSTALLATION FOR ELECTRIC CIRCUITS Filed May 9, 1922 2300 ms. Q 4

77 )NsULHT/D/ Low M54 T/Nc WIVT I INVENTO/c' BYL/Qm A TTORNEVS Patented June 2, 1925.

.JuNiT-Eo sTATEs PATENT OFFICE.

CrrAELEs E BENNETT, or DECATUR, GEORGIA, AssIGNOn. To BALT MANUEACTUEJN 1 COMPANY, 01 ATLANTA, GEORGIA, A CORPORATION or GEORGIA.

PROTECTIVE INSTALLATION FOR, ELECTRIC CI CUITS.

Application filed m a, 1922. Serial No. 559,612.

To all 'wlwm it may concern:

Be it known that I, GHARLEs E. BENNETT,

a 'citizen'of the United States of America, 1

residing at Decatur, in the county of Dekalb and State of Georgia, have invented certain new and useful Improvements in Protective Installations for Electric Circuits, of which the following is a specification.

My invention relates to a protective installation for electric circuits, and particularly for low voltage distribution circuits. The object of my invention is to provide an installation which will effectively protect such a crcuit, not only against over-potentials at low frequency, but also against high frequency potentials.

In the accompanying drawings Fig. 1 is a dlagram of an illustrative installation; and

Fig. 2 is a vertical section through a niore or less diagrammatically shown practical embodiment, in one form, of the protective elements of the installation.

In low voltage lines- -say lines of from 30 to 300 volts such as are used for house wiring, signal circuits, telephone lines, etc.-

there is a constant fire hazard, as well as danger of injury to the installation andto the equipment served thereby, through-the imposition thereon of overotentials at low frequency. There, is also anger of injury to the line and its equipment through the imposition of high frequency potentials thereon. While 'it is common to provide such circuits with lightning arresters to' protect them against high frequency discllliar es, these devices are not adapted *for t e frequency, and as a. result they are commonly destroyed at the very moment when the are most'needed.

'l he present invention provides aprote'ctive installationwhich, rst, is extremely sensitive to high frequency :ourrentand readily drains the line of boundcharges, sneak currents, impulses, etc., all having the characteristics" of high frequency but low ischarge of over-potentials at normal power line on the conductors .of the low voltage circuit.

While an installation embodying this underlying thought may be effected in various ways, I have illustrated a simple and efiicient construction which comprises a liquid insulated gap arranged in a ground line and adapte to drain the circuit of over-potentials at low frequency, together with an associated vacuum gap arranged in a shunt around the first-mentioned gap, and serving to drain the line of high frequency potentials.

Such an installation is indicated in Fig. 1, in which the high voltage power line 3 is stepped down by the transformer 4 to load 6 is arranged. Across the low voltage line 5 are connections 7 and 8 leading to the protective ground line 9, but separated.

therefrom by an Oil grip 10 and a vacuum gap 11 arranged in pa lel, the ground terminals of the two gaps being the same, or connected together by a bridge conductor 12. Fuses 13, or other suitable rotective equipment, ma be arranged in t e power line 3.

In the arger scale drawing of Fig. 2, the leads 7 and 8 are connected to terminal bars 14 and 15, which project downward into the tank 16- and enter an insulating fluid 17, such as Oil, with which the tank is suitably filled. At the lower end of the terminal rods are electrodeiterminals 18 and 19, each spaced" a slight distance from the submerged round electrode 20 to which the ground fine 9 is connected. The terminals 18 and .19 are preferably of metal of low melting '1 26, which is connected b the bridge 12 to the ground terminal 20 o the oil gap.

A vacuum gap of the present. characte;

is an ideal protector for,h1gh frequency impulse or potentials of steep wave front, the partial vacuum being a fair conductor for potentials of these high frequencies. There;

fore high frequency impulses due to lightning discharges, etc., induced on the distribution line 5, are readily drained across the gap of the vacuum tube 26 th the ground inc 9.

On the other hand, the oil or other medium of high resistance to impulse voltages at low frequency does not readily permit the passage of high fre uency currents to the ground terminal 20. e insulating character of oils and certain other soft insulations, is remarkably higher with respect to high frequency current than with respect to low frequency current. For instance, one inch of oil is readily punctured by a low frequency spark of a few inches, whereas the same one inch of oil will withstand successfully a 50 inch high frequency spark equivalent. The spacing ofthe termmals 18 and 19 from the ound terminal 20 may therefore be so predetermined that the oil will break down when a low frequency current in line 5 reaches a dangerous potential, while preventing the flow of low voltage high frequency current.

Therefore, the vacuum gap drains' the line of potentials of high frequency, or steep wave front, character, but of relatively small voltage, and prevents the submerged oil gap from operating under these conditions. However, the oil gap will break down readily when heavy voltage is imposed on the low voltage circuit, and immediately drains the line of the excess'potential. Inasmuch as the discharge of the low frequency, over-potential results in the generation of heat at the gap, this incident to a discharge is utilized to fuse the electrodes 18, 19 and 20, or certain thereof, which are of low melting point. The melting of the terminals causes a short circuit of the line 5, which will have the immediate effect of blowing the fuses 13 in the power line 3, and thus prevent injur to the transformer 4.

The underlying t ought of the invention is thus the combination of the two line draining connections, viz, one of extreme sensitiveness to high frequency impulses,

and the other of sufiicient sensitiveness to low frequency to protect the line againstover-potentials, but which is repellent to currents of high frequency and causes the latter to drain off through the agency proyided for that purpose. Since this thought is susce tible of embodiment in various forms 0 apparatus, I do not limit my invention to the particular construction shown, but claim 1. In a protective ground connection for an electric circuit, a gap sensitively responsive only to low fre uency over-potentials 'and comprising rea ily fusible electrode means serving by fusion on discharge across the gap to form a continuous meta lic path to ground, a shunt across the gap, and in said shunt a gap sensitively responsive only sitive only to low frequency over-potentials,-

in combination with a shunt across the ap terminals, and in said shunt a gap having terminals spaced by a vacuum to render the gap sensitive only to high frequency potentials. v

3. In a protective ground connection for an electric circuit, a gap sensitively responsive only to low frequency over-potentials and having relative closely spaced terminals I and a liquid insulator interposed between the same, in combination with a shunt across said gap, and in said shunt a gap sensitively responsive only to high frequency potentials and having terminals spaced relatively far apart in a vacuum.

4. In a protective ground connection for an electric circuit, gap terminals spaced by an insulating liquid to rendersaid gap sensitive only to low frequency over-potentials, and means at the gap for short circuiting the line on heavy potential discharge across the gap, in combination with a shunt across the gap terminals, and in said shunt a gap having terminals spaced by a vacuum to render the gap sensitive only to high frequency potentials.

5. In a protective ground connection for an electric circuit, gap terminals spaced by an insulating liquid to render said gap sensitive only to low frequency over-potentials, certain of said terminals bein of low-fusing-point and being so spaced t at on fusion 'a conducting bridge is formed which shortcircuits the line. I

6. For use in a protective round connection, a pair of terminals apted to be connected across theline, a terminal ada ted to be connected to ground, and in-para lel between the Iineterminals and the ground terminal, two gap arrangements,- one sensitive only to high frequency the other sensitive only to w frequency over-potentials and comprising a grounded electrode, and spaced therefrom a pair of electrodes connected respectively to the 11118 terminals, certain of said electrodes being of metal of low meltin point, serving by fusion on the flow 0 grounded electrode and either of the terminal electrodes to establish a direct metallicground connection which short clrcults the gap sensitiveto low frequency overpotentials.

7. For use in a protective round conmotion, a pair of terminals a apted to be connectedacross the line, a terminal adapted to be connected to ground, and in parallel between the line terminals and the ground terminal, two gap arrangements, one sensipotentials, and a current between the line terminals and the ground terminal, cer- 10 tain of said terminals being of low fusing point and serving on fusion by discharge across the gap, to short-circuit and ground the line.

In testimony whereof I have signed my 15 name to this specification.

CHARLES E. BENNETT. 

